The principle is already known of making a parity check for protecting stored data in data processing units. In this process an additional signal is added to a long uniform sequence of information characters. The value of this additional signal is chosen in a predetermined manner so that with it the number of information characters with a prescribed value above the total unit of the sequence of information characters always reaches an even value or always reaches an odd value. Then with the parity check a parity signal present in a character sequence is compared with a parity signal newly formed by the same process. If the two parity signals do not agree, then there is a parity error, which means that one or more information characters are invalidated by error influences.
When such a parity error occurs, the operating system of a data processing unit can initiate an error routine or can trigger the central processing unit in such a way that the latter interrupts the running of a program which is already started running. In this case, the program may for example remain at a definite address.
A special application case of the parity check is checking the programs in a data processing unit for program mistakes; i.e., program debugging. A data processing unit contains an operating system which controls its basic sequence of operations, that is, the mode of operation of its operational units. Moreover, different user programs can be provided or input which will fulfill the respective application purpose of the data processing unit and thus solve user problems. Such user programs must be checked for program mistakes in the data processing unit under control by the operating system in order to prevent malfunctions from occurring in the running of the respective user program.
A program to be checked causes normalization of all of the memory cells to be used by it in the working memory of the data processing unit before it is actually completed. This means that initial information is written into these memory cells which prevents the memory cells, after the data processing unit is connected in, from still containing information from programs previously run which are not relevant for the running of the program to be checked. However, if the program to be checked contains a program mistake, then the memory cells of the working memory are not normalized in order. Then when in the course of running the program a read access to a non-normalized memory cell occurs, false information is then read out. However, it is not possible to immediately identify the program mistake causing it. Since program mistakes of this type are the most frequent but their analysis is very difficult, since they cannot be identified immediately, there is a need for a checking process which avoids this disadvantage.
Accordingly, it is the objective of the invention to supply a process for checking a program which will make possible the immediate identification of program errors which appear as a defective or erroneous normalization of memory cells.